Many window sash counterbalance systems rely on locking of carrier shoes in place when a sash tilts. Otherwise, tilting a sash removes some of its weight from the counterbalance system, which would raise the sash shoes if they were not locked in place.
A multitude of arrangements have been devised for locking carrier shoes in place in shoe channels when a sash tilts. Many of these involve cams that are turned when the sash tilts so that the cams move locking elements that make the carrier shoe either wider or thicker so that it is no longer free to move vertically in a shoe channel.
Many such locking arrangements are problematic and not completely reliable. One difficulty with locking shoes is variations in the dimensions of the channels in which the shoes must lock. This can be caused by temperature and speed variations in the extrusion processes that form shoe channels. Any device for satisfactorily locking sash shoes must be able to accommodate the unavoidable variations in shoe channel dimensions. Another challenge is that shoe locks must often rely on an interengagement between low friction resinous materials of both the shoe and the channel. Finally, the cost of a shoe locking device is always an important factor, since window counterbalance systems are highly competitive in cost, as well as performance. In spite of the many suggestions for shoe locking arrangements, completely satisfactory and reliable locking systems remain elusive.